1. Field of the Invention
The present invention relates to a technology for manufacturing an optical fiber preform and an optical fiber for optical communication.
2. Description of the Related Art
Currently, a demand for reducing a manufacturing cost of an optical fiber is ever increasing, in addition to an improvement of optical transmission characteristics of the optical fiber.
A method of manufacturing the optical fiber includes a vapor-phase axial deposition (VAD) method, a modified chemical vapor deposition (MCVD) method, an outside vapor deposition (OVD) method, a plasma chemical vapor deposition (PCVD) method, a sol-gel method, a rod-in-tube (RIT) method, and a combination of these methods.
The above manufacturing methods are already developed into a mature technology, and there is little room for an improvement. For this reason, it is not easy to realize more cost reduction for manufacturing the optical fiber. Furthermore, because a manufacturing capability of a majority of optical fiber manufacturing equipments, which are installed by an investment in plant and equipment actively performed around the year 2000, is in surplus, it is extremely difficult to make an additional investment or development. As a result, these old manufacturing facilities are now one of the major impediments against a reduction of the manufacturing cost.
Because the current environment for manufacturing the optical fiber is in the above situation, it is desired to work out a method of manufacturing the optical fiber, which can reduce the manufacturing cost by proficiently combining the conventional technologies, while utilizing the existing manufacturing facilities as much as possible.
For example, when forming a cladding layer that makes up a major portion of the optical fiber, a method of forming a porous layer by depositing particles of a silica glass that is created by a vapor phase synthesis, such as the OVD method, around a core rod, and manufacturing a transparent glass layer via a thermal treatment of the porous layer is an excellent method of manufacturing a large-size optical fiber preform with a low cost and a high quality.
In general, a vitrification equipment, which makes the porous layer formed by the vapor phase synthesis, such as the OVD method, the transparent glass layer via the thermal treatment, employs a heating furnace having a muffle tube made of the silica glass. However, the muffle tube is softened at a temperature close to 1,600° C., which is necessary for making the porous layer into the transparent glass, and is apt to be deformed. In particular, a large-diameter muffle tube is required for making a large-size optical fiber preform, and such a large-diameter muffle tube has a problem of durability. To cope with this problem, a method of manufacturing a translucent glass preform and drawing the optical fiber is disclosed in Japanese Patent No. 2565712.